High voltage supply for compact radiation generator

ABSTRACT

Disclosed is a radiation logging tool, comprising a tool housing; a compact generator that produces radiation; a power supply coupled to the compact generator; and control circuitry. Embodiments of the compact generator comprise a generator vacuum tube comprising a source generating charged particles, and a target onto which the charged particles are directed; and a high voltage supply comprising a high voltage multiplier ladder located laterally adjacent to the generator vacuum tube. The high voltage supply applies a high voltage between the source and the target to accelerate the charged particles to a predetermined energy level. The compact generator also includes an electrical coupling between an output of the high voltage supply and the target of the generator vacuum tube to accommodate the collocated positions of the generator vacuum tube and the high voltage power supply.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 61/261,398, filed provisionally on Nov. 16, 2009. Thisapplication also claims priority to and the benefit of U.S. ProvisionalApplication No. 61/261,394, filed provisionally on Nov. 16, 2009.

TECHNICAL FIELD

The invention relates generally to borehole/oilfield electronicradiation generators.

BACKGROUND

Ionizing radiation has been used in the oilfield for well over half acentury to enable measurements not possible with other formation probingtechniques (such as electrical resistivity, acoustic, electro-magnetic,NMR, etc.). The broad types or kinds of ionizing radiation includecharged _(p)articles, X-rays, gamma-rays, and neutrons.

Historically, the first method employed to produce ionizing radiationfor oilfield applications involved radioactive sources. Theseradioactive sources (e.g. ¹³⁷Cs, ²⁴¹Am, ²⁵²Cf, ²³⁸Pu, etc.), werechosen, based on the application, for their emitted radiation type andin quantities sufficient for the desired intensity. Few naturallyoccurring radioactive sources emit neutrons during their decay,generally as a consequence of spontaneous fission. One example of aradioactive material with a significant probability for spontaneousfission is ²⁵²Cf. While ²⁵²Cf is used in some applications, its shorthalf-life of only 2.6 y is generally considered a drawback. Many neutronsources rely on the generation of neutrons through a nuclear reaction,typically the ⁹Be(alpha, n)¹²C reaction. Such sources consist of analpha emitter, such as ²⁴¹Am or ₂₃₈Pu, mixed with Be-powder. The smallnuclear reaction probability necessitates the use of a large amount ofradioactive alpha emitter to generate a sufficient neutron output.

The small size and simplicity of the source housing, the reliability ofcontinuous radioactive decay, and the typically low purchase cost makesuch radiation sources very attractive. The shortcomings, however,include limited radiation output, (particularly for neutron sources),issues of radiation safety because the source remains ON at all times,as well as the increase in the cost of the material and its disposal,the accountability (tracking/inventory requirements) and, for some like²⁵²Cf, a short useful life. The absence of the ability to fast pulseON/OFF a chemical source also limits the measurements that can be made.

To overcome these disadvantages of radioactive sources, electronicradiation generators were developed over the last several decades.Electronic radiation generators function by accelerating and collidingcharged particles inside of sealed tubes to induce radiation-producingreactions. The two approaches to accelerating charged particles are touse linear accelerators (e.g., Linac) or circular accelerators (e.g.,cyclotron). The linear accelerators employed in the oilfield, and ofrelevance to this particular invention disclosure, areelectrostatic-based. Linear accelerators operate by creating asufficiently large voltage difference between a source of chargedparticles and a target. Once the source is turned ON, charged particlesentering the region of high voltage (HV) are accelerated to thepredetermined desired energy and made to collide with a suitable targetmaterial to produce radiation. If either the ion source or the voltageare turned OFF, the radiation stops. The accelerated charged particlescan be electrons, protons and heavier ions, which will generate ionizingradiation when they impinge on the target.

The intensity and energy of the radiation produced depends on the highvoltage used to accelerate the charged particles. Typically, the higherthe “high voltage” is, the higher the energy of the charged particles.High voltage power supplies capable of producing from tens to hundredsof kilovolts are desirable. A common power supply design employed isbased on the Cockcroft-Walton voltage multiplier, also known as theGreinacher multiplier. Such a voltage multiplier includes an array ofstages in series, shaped like a ladder, with each stage 100 comprising apair of capacitors 102 and a pair of diodes 104 (see FIG. 1).

During each ON cycle, the A/C current supplied by the step uptransformer 106 charges each respective capacitor 102, according to therectification provided by the diodes 104. The maximum admissible voltagefor each stage 100 depends on the capacitor and diode component ratingsas well as on the AC voltage provided by the secondary of the step uptransformer 106. To achieve a given high voltage, a ladder isconstructed such that, to first order (i.e., neglecting losses, etc.),enough stages are used to sum up to the predetermined desired voltage.The high voltage output 108 of the series of stages, or ladder, can thenbe fed to a voltage divider (bleed resistors, discussed in furtherdetail in related application Ser. No. 61/261,398, non-provisionalapplication filed concurrently herewith, to obtain a measurement of thehigh voltage fed out to neutron generator tube.

One electrical constraint for the design of high voltage power suppliesis that electrical hold off between high voltage and ground must besufficient over the whole length of the power supply. Given the limiteddiameter available in downhole tools, the stage component size must besmall enough to accommodate the necessary high voltage insulation over avery limited radial distance. Typically, the size of high voltagecomponents increases with their voltage rating. A tradeoff must be madebetween the stage (and component) size and the necessary high voltageinsulation based upon the voltage difference to neighboring groundpotential. For borehole-size radiation generators, high voltagecapacitor ratings are typically on the order of around a few kilovolts(e.g., five kilovolts). Typically, multi-layer capacitors are used toachieve the required capacitance, and are generally rectangular orbox-shaped with radial leads. Thus, typically to produce from tens tohundreds of kilovolts requires a power supply with several dozens ofstages. When the stages are placed end to end, this adds up to asignificant overall length for the power supply.

Currently, the most frequently employed electronic radiation generatorin the oilfield is the pulsed neutron generator (PNG), as shown in FIG.2. Aside from the supporting low voltage electronics (current andpulsing supplies), a PNG comprises a high voltage power supply 202(comprising comprises the step up transformer and the voltagemultiplier), a sealed neutron generator tube 204 and a high voltageinsulation system 206. The sealed neutron generator tube 204 includes anion source 208 which is at or near ground potential, and separated froma target 210 by an HV acceleration column. The configuration of a PNG isgenerally cylindrical, as dictated by the borehole geometry, with thetarget 210 of the neutron generator tube 204 in contact with the highvoltage end of the high voltage power supply 202. The necessary highvoltage insulation (solid, liquid and/or gaseous or any combinationthereof) surrounds the neutron generator tube 204 and high voltage powersupply 202, with the entirety being contained in a hermetically-sealedpressure housing 212 (not to be confused with an overall tool pressurehousing of a tool in which the present generator is utilized).Preferably, the pressure housing is cylindrical in shape. Theconventional arrangement of these components is in a rectilinearconfiguration, where the high voltage power supply 202 is connected inline (on one axis or close to on one axis) to the high voltage end ofthe neutron generator tube 204 (see FIG. 2), frequently with aprotective surge resistor 214 therebetween. The typical length of such agenerator is often in the range from about 36 to 48 inches, or more.

The considerable length of a typical PNG configured as in FIG. 2 canimpose significant limitations on the tool design and consequently therange of measurements possible to obtain. Because of the length of thehigh voltage power supply in the typical PNG, only the neutron generatortube end affords close proximity to the radiation source (i.e., thetarget). For most practical purposes, this limits detector placement toa single axial direction relative to the target. If a tool is intendedto perform multiple different measurements with the radiation providedby the electronic source, it is likely that not all the requireddetectors can be placed at the optimal axial and/or radial position. Ashort radiation source would allow detector placements axially on bothsides of the radiation source, and thus a better independentoptimization of multiple measurements through optimal placement ofmultiple detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a design of a prior art Cockcroft Walton voltage multipliergenerator.

FIG. 2 shows a conventional prior art layout of a Pulsed NeutronGenerator for downhole use.

FIG. 3 shows a layout of a compact generator with collocated linear highvoltage power supply and a neutron generator tube in one embodiment inaccordance with the present disclosure.

FIG. 4 shows a compact generator with an external step-up transformer inanother embodiment in accordance with the present disclosure.

FIG. 5 shows a high voltage turn-around from end of ladder back to theneutron generator tube in another embodiment in accordance with thepresent disclosure.

FIG. 6 shows a “split-and-flip” voltage multiplier ladder providing areduced electrical potential at the ends allowing the shortest axialdistance to ground in another embodiment in accordance with the presentdisclosure.

FIG. 7 shows a schematic layout of a neutron generator with a collocated“split-and-flip” voltage multiplier ladder in another embodiment inaccordance with the present disclosure.

FIG. 8 shows an alternate implementation of “split and flip” ladder inaccordance with the present disclosure.

FIG. 9 shows a fold-back high voltage supply.

FIG. 10 shows a fold-back high voltage supply with intermediateelectrode in insulating support in another embodiment in accordance withthe present disclosure.

FIG. 11 shows a compact generator with insulating support with coolantchannel in another embodiment in accordance with the present disclosure.

FIG. 12 shows a neutron generator with collocated voltage multiplierladder connecting an intermediate potential from the ladder to theintermediate electrode of the neutron generator tube in anotherembodiment in accordance with the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present disclosure. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments are possible.

In light of the above discussion, it would be advantageous to reduceradiation generators, including a PNG, to as short and compact a size aspossible.

Most pulsed neutron generators generate neutron radiation through thereaction of energetic deuterium with tritium (d-T) reaction. However,other known reactions, like d-D and t-T can be employed as well. X-raysare generated by directing energetic electrons onto a target. Thescattering of the energetic electrons on the target leads to theemission of Bremsstrahlung x-rays and x-rays characteristic of thetarget material.

The present disclosure discusses solutions for a compact generatorcollocated with a linear multi-stage power supply, such as aCockcroft-Walton (CW) power supply. In a most preferred embodiment ofthe present disclosure, a multi-stage high voltage power supply, such asa Cockcroft-Walton styled voltage multiplier, would be employed, howeverparallel work on single-staged voltage multipliers indicates thatsingle-staged voltage multipliers are also contemplated in the presentdisclosure, as any other axially collocated supplies could be used. Anembodiment of a pulsed neutron generator collocated with its highvoltage power supply is shown in FIG. 3. The neutron generator 300 isenclosed in a hermetically sealed housing 302 which contains the step-uptransformer 304, which is powered by an external voltage source (notshown in FIG. 3). A stepped up high voltage on the order of severalkilovolts powers the voltage multiplier 306, which appears above theneutron generator tube 300 in FIG. 3, though for practical purposes, thevoltage multiplier 306 is situated substantially side-by-side,collocated with the neutron generator tube 300, thereby shortening theoverall length required for the pulsed neutron generator. The neutrongenerator tube 300 contains an ion source 308 on one side and a target310 on the other side on which accelerated beam particles impinge. Theion source 308 and the target 310 are separated by an acceleratingregion, which is surrounded by the insulating wall of the neutrongenerator tube 300. The wall of the neutron generator tube 300 maycomprise, for example, a ceramic. The high voltage generated by thevoltage multiplier 306 is fed to the target 310 through a protectiveresistor 314 (i.e., a surge resistor). The end of the neutron generatortube 300 having the target 310 is supported by an axial insulator 316that may also serve as the mechanical support for the protectiveresistor 314. A high voltage turn-around 318 couples between the outputof the high voltage multiplier 314 to the input of the neutron generatortube 300 (often through a protective surge resistor 314).

Referring now to FIG. 4, in an alternative embodiment, the step uptransformer 404 may be located outside of the generator housing 402 at asecond location to allow a further reduction of the overall length ofthe pulsed neutron generator, wherein the step-up transformer 404 iscoupled to the input of the high voltage multiplier 306 via high voltagewires 420 passing into the housing 402 via high voltage feed-throughpassages 422. One effect of such a configuration is that high voltagesof 10 kV or more have to be connected over a distance of several feet tothe generator. In the limited space of a downhole tool, such a may poseproblems due to the required diameter of the wires for such a connection(including high-voltage insulation) as well as noise induced by highvoltage wiring. Despite these factors, locating the step up transformer404 outside of the generator housing is a possible embodiment.

Referring now to the embodiment of FIG. 5, the electrical connectionfrom the end of the voltage multiplier 506 to the target 510necessitates a turn-around 503 of the high voltage line. Given theproximity of the turn-around 503 to the ground plane at the end (shownright side of FIG. 5) of the generator housing 502, resulting electricalfields have the potential to lead to uncontrolled arcing. Theturn-around 503 may therefore be protected by a corona shield 505positioned so as to reduce electrical field stresses. A possibleapproach is shown in detail in FIG. 5.

As shown in FIG. 5, the target 510 end of the neutron generator tube 500are shown contained in the wall 501. Adjacent to the neutron generatortube 500 is the voltage multiplier 506 with a high voltage line 503exiting the voltage multiplier 506. The corona shield 505 is providedabout the high voltage line 503 (and including about the surge resister514). The corona shield 505 surrounding the high voltage wire 503 (orother components at high electric potential) serves to reduce theelectric field around such components or wires. Typically, it is ametallic conductor (other conducting materials can be used as well) witha highly polished surface and a large radius of curvature. The increasedradius of curvature and the absence of surface irregularities (sharpedges and rough surface) minimize the electric field at the surface andreduce the probability of electron emission from the surface, which maylead to an electrical breakdown of the insulation. Additionally, thecorona shield 505 may be coated by a thin insulating layer to furtherreduce the probability of electron emission from the surface.

An alternate approach for reducing electrical stress at the turn-aroundis to “split-and flip” the voltage multiplier back on itself. Theschematic in FIG. 6 illustrates this approach, which affords a reductionin the turn-around voltage stress, but at the expense of moving theelectrical field stress midway through the voltage multiplier and addingto the complexity by feeding back high voltage wires. FIG. 6 shows thesplit 600 of the voltage multiplier at a certain stage, with feedbackfrom the end back towards the middle, with the HV output at anintermediate of the voltage multiplier.

FIG. 7 shows the schematic layout of a neutron generator employing a“split-and-flip” voltage multiplier as outlined in FIG. 6. The region ofhigh electric stress is indicated. An alternate embodiment of the“split-and-flip” approach is shown in FIG. 8 where the split divides thehigh voltage multiplier into a first section 806A located outside, butproximate, the sealed neutron generator tube 804, and a second section806B located in-line with the sealed neutron generator tube 804 in asupport 850.

Yet another approach consists of folding the voltage multiplier itselfand bringing it back as shown in FIG. 9. This means that some of themultiplier stages are located underneath the main support of the highvoltage supply that contains the majority of the multiplier stages. Thisresults in a similar reduction of the high voltage stresses at theposition closest to the end of the generator as in FIG. 7. In bothcases, the high voltage at the far end of the HV supply is lower thanthe maximum high voltage. This may allow a reduction in the length ofthe insulating mechanical support and as a consequence a shorter overalllength of the PNG. In this case, the portion of the voltage multiplierthat is folded back requires additional room underneath the supply.

For both embodiments, an additional improvement can be accomplished bymaking sure that an axial plane coinciding with the fold of the voltagemultiplier is at the same electrical potential as the folding point inthe voltage multiplier. This can be accomplished by inserting aconductive plane into the insulating support that connects the end ofthe generator tube to the end of the generator housing. The additionalelectrically conducting plane is shown in FIG. 10.

FIG. 8 can also be used for the folded voltage multiplier. Thesupporting insulator (shown in FIGS. 7-12) extending from the target endof the generator tube to the end of the generator housing is includednot only for mechanical strength but also as a heat conducting medium toremove heat from the target. Typically, electrical insulators are poorthermal conductors and the Alumina ceramic is no exception to this rule.However, Aluminum nitride (AlN) is a much better conductor of heat whileat the same time being a good electrical insulator. The insulatingmechanical support 730, 830, etc. could therefore be manufactured ofAluminum nitride. Matters of heat conduction are critical for thereliable operation of the PNG at elevated temperature. A traditional d-Tgenerator with a titanium-based target will stop working when the targettemperature exceeds 250° C. At an ambient temperature of 150 or 175° C.,the target temperature can reach or exceed 250° C. unless there is aproper heat sink. The insulating mechanical support, in particular ifmade of AlN, offers an excellent solution to this problem. Alternativelyor additionally, one could envisage circulating an insulating fluid (gasor liquid) through the insulating support in order to evacuate heat(FIG. 11)

The ceramic support can be made in such a way that the heat evacuationfrom the CW voltage multiplier is facilitated to reduce the temperatureseen in particular by the diodes but also by the capacitors.

Collocation of the voltage multiplier with the generator tube hasadditional advantages. It is feasible to tap off from intermediatestages of the CW generator to provide a well defined potential to one ormore intermediate electrodes of the generator tube. Radiation generatortubes with multiple intermediate electrodes are described in more detailin the concurrently pending U.S. patent application Ser. No. 12/618,821filed Nov. 16, 2009, entitled “Electrode Configuration for DownholeNuclear Radiation Generator”. An example of such an arrangement using anintermediate-electrode-neutron-tube is shown in FIG. 12. In order toprotect the voltage multiplier from transients due to arcs to theintermediate electrode, it may be necessary to add a protective (surge)resistor between the intermediate voltage multiplier point and theintermediate electrode (not shown in FIG. 12). It is evident that theconcept can be expanded to multiple electrodes with multipleintermediate potentials.

While the invention has been disclosed with respect to a limited numberof embodiments, those skilled in the art, having the benefit of thisdisclosure, will appreciate numerous modifications and variationstherefrom. It is intended that the appended claims cover suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A compact device that generates radiation,comprising: a generator vacuum tube comprising: a source generatingcharged particles, and a target onto which the charged particles aredirected; a high voltage supply comprising a high voltage multiplierladder located at least partly side-by-side and substantially adjacentto the generator vacuum tube, the high voltage supply being configuredto apply a high voltage between the source and the target to acceleratethe charged particles to a predetermined energy level; and an electricalcoupling between the high voltage supply and the target of the generatorvacuum tube, wherein the electrical coupling comprises a high voltageturn-around comprising a split and flip that accommodates an electricalfield stress caused by the high voltage multiplier ladder being locatedside-by-side adjacent to the generator vacuum tube, wherein the splitand flip reduces electrical field stress by a split in the high voltagemultiplier ladder such that the high voltage multiplier ladder comprisesa first portion and a second portion and the first portion of the highvoltage multiplier ladder is turned directionally back toward thetarget.
 2. The compact device according to claim 1, the generatedradiation comprising neutron radiation.
 3. The compact device accordingto claim 1, the generated radiation comprising x-rays.
 4. The compactdevice according to claim 1, the generated radiation comprisinggamma-rays.
 5. The compact device according to claim 1, wherein thecompact device measures an overall length of less than about twenty (20)inches in length.
 6. The compact device according to claim 1, furthercomprising a corona shield covering the high voltage turn-around toreduce the electrical field stress.
 7. The compact device according toclaim 1, further comprising a corona shield about the split.
 8. Thecompact device according to claim 1, further comprising an electricallyinsulating mechanical support that is located proximately to the targetand provides mechanical support to an end of the generator vacuum tube,the electrically insulating mechanical support comprising a conductorpositioned perpendicular to the axis of the electrically insulatingmechanical support.
 9. The compact device according to claim 8, furthercomprising a corona shield that reduces electrical field stress at thehigh-voltage turn-around.
 10. The compact device according to claim 9,wherein the conductor is operatively coupled to the corona shield. 11.The compact device according to claim 8, wherein the electricallyinsulating mechanical support comprises Aluminum Nitride.
 12. Thecompact device according to claim 1, wherein the generator vacuum tubecomprises at least one intermediate electrode located between the sourceand the target and operatively coupled to an intermediate-voltage pointlocated between multiplier stages at an intermediate potential betweenan input voltage and an output voltage of the high voltage multiplierladder.
 13. The compact device according to claim 12, further comprisinga protective surge resistor coupled between the intermediate electrodeand the intermediate-voltage point between the multiplier stages. 14.The compact device according to claim 12, further comprising a diodecoupled between the intermediate electrode and the intermediate-voltagepoint-between the multiplier stages.
 15. The compact device according toclaim 8, the electrically insulating mechanical support furthercomprising an internal flow path configured for circulation of aninsulating cooling fluid.
 16. A radiation logging tool, comprising: atool housing; a compact generator that produces radiation through areaction of energetic charged particles accelerated in a DCelectrostatic field with a target on which the charged particlesimpinge; a power supply operatively coupled to the compact generator;control circuitry operatively coupled to the compact generator; whereinthe compact generator comprises: a generator vacuum tube comprising: asource generating charged particles, and a target onto which the chargedparticles are directed; a high voltage supply comprising a high voltagemultiplier ladder located at least partly side-by-side and substantiallyadjacent to the generator vacuum tube, the high voltage supply beingconfigured to apply a high voltage between the source and the target toaccelerate the charged particles to a predetermined energy level; and anelectrical coupling between the high voltage supply and the target ofthe generator vacuum tube, wherein the electrical coupling comprises ahigh voltage turn-around comprising a split and flip that accommodatesan electrical field stress caused by the high voltage multiplier ladderbeing located side-by-side adjacent to the generator vacuum tube byreducing the electrical field stress at the high voltage turn-around,wherein the split and flip and reduces electrical field stress by asplit in the high voltage multiplier ladder, wherein the high voltagemultiplier ladder comprises a first portion and a second portion and thefirst portion of the high voltage multiplier ladder is turneddirectionally back toward the target, wherein the voltage level at anend of the first portion and a beginning of the second portion is lowerthan an output voltage of the high voltage multiplier ladder.